The present invention relates in general to fiber Bragg gratings, and in specific to methods and apparatuses for producing masks that are used to create fiber Bragg gratings.
Normal optical fibers are uniform along their lengths. A slice from any one point of the fiber looks like a slice taken from anywhere else on the fiber, disregarding tiny imperfections. However, it is possible to modify fibers in such a way that the refractive index varies regularly along their length. These fibers are called fiber Bragg gratings (FBG). The periodic refractive index variation causes different wavelengths of light to interact differently with the fiber, with certain wavelengths being reflected and certain wavelengths being transmitted.
Whenever there is a change in the index of refraction within the fiber, there is a slight reflection from the transition. In an FBG there are many of these slight reflections. The locations of these reflections are arranged such that the reflections all interfere with each other to create a strong reflection at a certain wavelength. This is the so called Bragg condition, and is satisfied when the wavelength of light is equal to twice the period of the index modulation times the overall index of refraction of the fiber. Light that does not meet this Bragg condition will be transmitted.
FBGs can be used to compensate for chromatic dispersion in an optical fiber. Dispersion is the spreading out of light pulses as they travel on the fiber. Dispersion occurs because the speed of light through the fiber depends on its wavelength, polarization, and propagation mode. The differences are slight, but accumulate with distance. Thus, the longer the fiber, the more dispersion. Dispersion can limit the distance a signal can travel through the optical fiber because dispersion cumulatively blurs the signal. After a certain point, the signal has become so blurred that it is unintelligible. The FBGs are used to compensate for chromatic (wavelength) dispersion by serving as a selective delay line. The FBG delays the wavelengths that travel fastest through the fiber until the slower wavelengths catch up. The spacing of the grating is chirped, varying along its length, so that different wavelengths are reflected at different points along the fiber. These points correspond to the amount of delay that the particular wavelengths need to have so that dispersion is compensated. Suppose that the fiber induces dispersion such that a longer wavelength travels slower than a shorter wavelength. Thus, a shorter wavelength would have to travel farther into the FBG before being reflected back. A longer wavelength would travel less far into the FBG. Consequently, the longer and shorter wavelengths can be made coincidental, and thus without dispersion. FBGs are discussed further in Feng et al. U.S. Pat. No. 5,982,963, which is hereby incorporated herein by reference in its entirety. A circulator is used to separate the light reflected from the FBG onto a different fiber from the input. With a properly designed FBG, the group delay is a function of the wavelength of the reflected beam and has the desired shape to compensate for dispersion (group delay) accumulated in propagation through an optical communication transmission system. One practical problem encountered with such FBG devices is that the group delay fluctuates around the desired functional shape. This deviation shall be referred to as group delay ripple (GDR) and is generally deleterious to the quality of transmitted optical signals.
FBGs are typically fabricated in two manners. The first manner uses a phase mask. The phase mask is a quartz slab that is patterned with a grating. The mask is placed in close proximity with the fiber, and ultraviolet light, usually from an ultraviolet laser, is shined through the mask and into the fiber. As the light passes through the mask, the light is primarily diffracted into two directions, which then forms an interference pattern in the fiber. The interference pattern comprises regions of high and low intensity light. The high intensity light causes a change in the index of refraction of that region of the fiber. Since the regions of high and low intensity light are alternating, a FBG is formed in the fiber. See also Kashyap, xe2x80x9cFiber Bragg Gratingsxe2x80x9d, Academic Press (1999), ISBN 0-12-400560-8, which is hereby incorporated herein by reference in its entirety.
The second manner is known as the direct write FBG formation. In this manner two ultraviolet beams are impinged into the fiber, in such a manner that they interfere with each other and form an interference pattern in the fiber. At this point, the FBG is formed in the same way as the phase mask manner. One of the fiber or the writing system is moved with respect to the other such that the interference pattern is scanned and the fiber exposed. Note that the two beams are typically formed from a single source beam by passing the beam through a beam separator, e.g. a beamsplitter or a grating. Also, the two beams are typically controlled in some manner so as to allow control over the locations of the high and low intensity regions. For example, Laming et al., WO 99/22256, which is hereby incorporated herein by reference in its entirety, teaches that the beam separator and part of the focusing system are moveable to alter the angle of convergence of the beams, which in turn alters the fringe pitch on the fiber. Another example is provided by Glenn, U.S. Pat. No. 5,388,173, and Stepanov et al., WO 99/63371, both of which are hereby incorporated herein by reference in their entirety. Both teach the use of an electro-optic module, which operates on the beams to impart a phase delay between the beams, which in turn controls the positions of the high and low intensity regions.
Note that whichever manner is used, it is still difficult to manufacture FBGs. The period of the spacing of the index modulation of the fiber Bragg grating is typically about one-half micron. When a phase mask is used to fabricate an FBG, the period of the mask grating is chosen to be twice that of the FBG, or about 1 micron. Thus, the etched bars and spaces which comprise the phase mask are about five hundred nanometers in width. For example, one application of the FBG is dispersion compensation. In this application FBGs must have a chirp (a slow variation) in the period, which is typically a very small change (xcx9c1 nm) over the length of the FBG. Thus, the spacing would ideally need to be adjusted on a picometer scale to have the period change appropriately over the length of the grating. This presents a serious challenge in design of any grating writing system. Inaccuracies in forming the chirp can cause group delay ripple in the output of the FBG.
Each FBG writing manner has advantages and disadvantages when compared with each other. For example, the first manner, the phase mask manner, is relatively inflexible, as changes cannot be made to the mask. However, since the phase mask is permanent, the phase mask manner is stable, repeatable, and aside from the cost of the mask, relatively inexpensive to operate. On the other hand, the direct write manner is very flexible, and can write different gratings. However, this manner is less repeatable and is costly to operate. Also, the direct writing process must be very strictly controlled. Any variation will lead to differences between gratings. This is difficult because the coherence (i.e. the relative position of the index modulation on a nm scale) of the entire pattern, e.g. 20 cm or greater, must be maintained. Little changes in alignment, temperature, etc. can result in the loss of coherence.
Another problem with the phase mask manner resides in the fabrication of the masks. Masks are fabricated by lithographic or holographic techniques. More specifically, the exposure of the resist that coats the mask may be done holographically, as well as lithographically. In the lithographical method, a small beam (of width smaller than the minimum mask feature sizexe2x80x940.5 micron) is used to directly expose the resist with the desired pattern. In the holographic method, two large (large meaning having a beam section that is approximately the same size as the mask) beams are interfered with each other to produce a periodic intensity pattern that exposes the resist on the mask substrate. While this process is used for simple masks, it is limited in its capabilities since the phase fronts of the interfering beams cannot be easily varied arbitrarily. For complicated masks, containing phase shifts and complex (nonlinear) chirp functions, current art holographic methods are not effective and lithographic methods are preferred.
The mask slab is coated on its surface with resist, which is a light (photo) or particle (electron or ion) sensitive material. Under the resist, the slab may also be coated with a metallic layer (e.g. chrome) to assist conduction of charged particles away from the exposed regions. Regions or bars of the resist are illuminated by light or particle beams according to a desired pattern, which is generally an array of parallel bars along a straight line with precisely selected positions. This illumination causes chemical changes in the exposed regions of resist. The exposed resist can be preferentially removed from the slab by a chemical or plasma, which does not strongly affect the unexposed resist (or vice versa). After the preferential removal of the resist according to the desired pattern, the slab may then be etched by a different chemical or plasma, which preferentially etches the slab where the resist has been removed. The etched portions of the slab have a difference in thickness or height from the un-etched portions. When the etched (bars) and un-etched (spaces) portions are patterned to form an array along a substantially straight line, the differences in thickness form a phase grating. Thus, by etching an array of bars and spaces on the slab to form a grating, a phase mask is fabricated. Other lithography tools can directly etch the bars and spaces onto the mask rather than in resist. In another embodiment, these regions can have alternate transmittance properties, such as by the presence or absence of an opaque material (e.g. chrome), and thus form an amplitude grating. Note that in all these cases, the critical part of the fabrication is the exposure of the bars and spaces (or direct etching of the bars and spaces). The resulting mask is limited by the quality and precision of the exposure process.
Current lithographic techniques use segmenting to encode the chirp into the mask. Due to the limitations of the lithographic writing machines, the period of the grating cannot be continuously varied. Fortunately, the grating can be written as a series of butt-coupled uniform period gratings which approximate a grating with a continuously varying period in a stepwise manner. A first series of bars, e.g. 500 bars, are written at a first period. A second series of bars are written at a second period, which is slightly different from the first period, and so on, until the desired variation of period (chirp) is written into the entire mask. The lithographic machines typically have a scaling feature that allows a segment to be scaled in size to picometer accuracy. Thus, a first segment is written at a first scale, and then the segment is rescaled to a different scale, which is slightly different from the first scale, and so on, until the mask is completely written.
This solution might be adequate for creating the proper pitch, but still suffers from a positioning error that occurs when the position is changed to write subsequent segments. This type of error is known as a xe2x80x98stitching errorxe2x80x99. Thus, each time the machine is rescaled and repositioned for a different segment, another stitching error is added to the mask. This, in turn, introduces an error into the grating that is written into the fiber. These errors cause group delay ripple in the optical signal reflected from the FBG. Consequently, the prior art attempts to write as few segments as possible, thus minimizing the number of stitching errors. For example, a typical mask would need about 100-200 scaled segments to encode the chirp into the mask. Thus, the prior art would only write about 100-200 scaled segments.
Note that the current technology for lithography does have the capability to write continuous patterns (so called cursive writing) effectively without such stitching errors. However, this cursive writing cannot be used to make masks for chirped FBGs and/or FBGs with arbitrary phase shifts (positional shifts of the bars or spaces, or changes in the bar or space widths or period), without the introduction of stitching errors. This is because such cursive writing methods would not allow for resealing of the grating period along the length of the phase mask. In addition, the locations of the bars and spaces on the mask are limited to fit on an address unit grid which is much too coarse to allow the picometer scale positioning required of the varying grating period.
These and other objects, features, and technical advantages are achieved by a system and method system which uses current lithography tools to fabricate masks with greatly reduce the effects of stitching errors from re-scaling or re-positioning. The masks fabricated by the invention will generate the linear or non-linear chirp, and other phase shifts as desired, in the fiber Bragg grating (FBG) in the core of the fiber.
A first embodiment of the invention preferably uses a characteristic of stitching errors to compensate for the stitching errors themselves. Each stitching error is typically random. Some stitching errors are formed when the segments are too far apart, thereby having too wide a space between the end bars of the adjacent segments. Other stitching errors are formed when the segments are too close together, thereby having too narrow a space between the end bars of the adjacent segments. Consequently, error induced by one stitching error may be offset by another stitching error. The invention preferably takes advantage of the characteristic that the effective error introduced into the grating from the mask with the stitching errors is the root mean square (RMS) of the stitching errors, when averaged over a length determined by the characteristics of the FBG design. This averaging length is typically on the order of 1 mm, and thus since the mask period is about 1 micron, the averaging occurs over about 1000 periods of the mask. Thus, increasing the number of stitching errors, by increasing the number of segments, can increase the number of errors being averaged by the light passing through the FBG. This increases the population of stitching errors and normalizes the mean, by bringing the median value closer to the mean value of pool of stitching errors. Thus, the overall average is brought closer to zero or no error. In other words, this increase in the number of stitching errors tends to reduce the RMS of the effective net stitching error by averaging out the stitching errors, and hence reduces the group delay ripple of a FBG created from the mask. Thus, a mask with an increased number of stitching errors, so long as these additional errors occur over the effective averaging length of the FBG, produces a grating with a lower group delay ripple error. If 1000 such errors are introduced over the effective averaging length in the FBG, then the net effective stitching error should be reduced by about √{square root over (1000)} or about 30 times.
The first embodiment is preferably implemented in one of two ways. In the first way, each segment is split into a plurality of segments that have the same scaling. For example, assume the example of the prior art segments, wherein 200 segments are used to form a 10 cm long mask, with each segment having a slightly different scaling factor. Each segment may be further split into 4 segments for a total of 800 segments, with each sub-segment within a particular group having the same scaling factor. In the second way, the scaling factor is adjusted for each of the smaller segments. For example, assume the example of the prior art segments, wherein 200 segments are used to form a mask, with each segment having a different scaling factor, as compared with an adjacent segment. This scaling change is generally extremely small. For a typical application of a 10 cm grating with 200 segments, the period may change about 3 pm per segment, as compared to the nominal 1000 nm period, or a scale change of about 3xc3x9710xe2x88x926. Each segment may then be further split into 4 segments for a total of 800 segments, with each segment having a smaller change in scaling factor of about 7.5xc3x9710xe2x88x927, as compared with an adjacent segment.
A second embodiment uses continuous writing of the desired pattern. Instead of writing a series of scaled segments, the entire grating or mask is written in one cursive writing cycle at a single scale, i.e. one continuous single-scale segment. Thus, there should not be any stitching errors as the writing equipment is not stopped for resealing and re-alignment. In writing a grating pattern that includes a fine scale chirp, the desired size of the bars and spaces (i.e. the location of the edges) may not be achievable on the address unit or pixel grid required by the writing equipment. The invention has the bar and/or spacing lines moved or snapped to the nearest address unit or grid. The error of placement of bar edges would accumulate as a difference between the ideal and the desired pattern until the error at most equals one-half of a pixel width, and then the edge would snap to the next grid location. While the misalignment between the designed edges and the actual edges will induce many more errors in the resulting fiber Bragg grating than the current art of using rescaled segments, the effective net error is minimized by the averaging described above with regards to the first embodiment. For a uniform distribution of error up to xc2x1one-half of the grid spacing (or pixel size) p, the expected statistical RMS error for each edge placement is found to be about xc2x10.29 p. In this case, for example, with the current lithography tools, the pixel size is p=5 nm, and the expected RMS error of an edge is xcx9cxc2x11.5 nm. Since these errors occur at every edge, which are typically separated by xcx9c1 xcexcm, in the above example of an effective averaging length of xcx9c1 mm, xcx9c1000 edge errors are averaged and therefore the effective net averaged error is reduced by √{square root over (1000)}xcx9c30 and is thus only xcx9c50 pm.
Note that the above described inventive embodiments can also be used for FBGs that have a second periodic pattern superimposed on the basic pattern. In general this pattern is introduced as an amplitude pattern or periodic set of phase shifts (see for example, U.S. application Ser. No. 09/757,386 entitled xe2x80x9cEFFICIENT SAMPLED GRATINGS FOR WDM APPLICATIONSxe2x80x9d filed Jan. 8, 2001, the disclosure of which is hereby incorporated herein by reference). This pattern serves to sample the initial grating, creating duplicate reflective channels at a spacing dependent on the period of the secondary (sampling) pattern.
The foregoing has outlined rather broadly the features and technical advantages of the present invention in order that the detailed description of the invention that follows may be better understood. Additional features and advantages of the invention will be described hereinafter which form the subject of the claims of the invention. It should be appreciated by those skilled in the art that the conception and specific embodiment disclosed may be readily utilized as a basis for modifying or designing other structures for carrying out the same purposes of the present invention. It should also be realized by those skilled in the art that such equivalent constructions do not depart from the spirit and scope of the invention as set forth in the appended claims. The novel features which are believed to be characteristic of the invention, both as to its organization and method of operation, together with further objects and advantages will be better understood from the following description when considered in connection with the accompanying figures. It is to be expressly understood, however, that each of the figures is provided for the purpose of illustration and description only and is not intended as a definition of the limits of the present invention.